1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly a zoom lens system suitably used for an image pickup apparatus such as a video camera or a digital still camera.
2. Related Background Art
In recent years, with improvement in a function of an image pickup apparatus using a solid-state image pickup element, such as a video camera or a digital still camera, a compact zoom lens system having a high resolution has been desired as an image taking optical system used for the image pickup apparatus.
In general, as a compact zoom lens used for the image pickup apparatus using a solid-state image pickup element such as a CCD sensor of a video camera, a digital still camera, or the like, there has been known a zoom lens which includes a first lens unit with negative refractive power, a second lens unit with positive refractive power, and a third lens unit with positive refractive power which are disposed in order from an object side to an image side and in which an interval between the second lens unit and the third lens unit increases during zooming from a wide angle end to a telephoto end (for example, JP 07-052256 B (corresponding to U.S. Pat. No. 4,733,952)).
As another example, there has been known a zoom lens which includes a first lens unit with negative refractive power, a second lens unit with positive refractive power, and a third lens unit with positive refractive power which are disposed in order from an object side to an image side and in which an interval between the second lens unit and the third lens unit reduces during zooming from a wide angle end to a telephoto end (for example, U.S. Pat. No. 5,434,710).
As still another example, there has been known a zoom lens having three lens units including a first lens unit with negative refractive power, a second lens unit with positive refractive power, and a third lens unit with positive refractive power, which are disposed in order from an object side to an image side (for example, JP 10-213745 A and JP 2001-100098 A). The first lens unit includes three or more lenses and has at least one aspherical surface.
As yet still another example, there has been known a zoom lens which includes a first lens unit with negative refractive power, a second lens unit with positive refractive power, and a third lens unit with positive refractive power, which are disposed in order from an object side to an image side (for example, JP 11-052237 A (corresponding to U.S. Pat. No. 6,081,389) and JP 2000-009999 A (corresponding to U.S. Pat. No. 6,172,818)). The second lens unit is composed of a lens unit having positive refractive power and a lens unit having negative refractive power.
Besides the above examples, there has been known a zoom lens which includes a first lens unit with negative refractive power, a second lens unit with positive refractive power, and a third lens unit with positive refractive power, which are disposed in order from the object side to the image side (for example, JP 2000-137164 A and JP 2000-147381 A (corresponding to U.S. Pat. No. 6,243,213)). Focusing is performed using the second lens unit.
In recent years, with reduction in size of the image pickup apparatus and increase in the number of pixels of the image pickup element, a zoom lens which has a high optical performance and which is small in the entire lens system has been desired as a zoom lens used for a digital still camera, a video camera, and the like. In addition, it has been desired to record a high quality still image by the video camera. Therefore, a lens system which has a high optical performance but is small is required.
In general, with respect to a negative lead type zoom lens, in order to reduce the number of lenses in the entire lens system to simplify a lens structure and in order to obtain a preferable optical performance over the entire zoom range while attempting to widen a view angle, it is necessary to suitably set refractive power to each of the lens units, a lens structure of each of the lens units, a surface provided as an aspherical surface if the aspherical surface is used, and the like.